1. Field of the Invention
The present invention relates to an antenna for broad-band ultrahigh frequency or to broad-band ultrashort wave antenna which can be used as, for example, a hand-carried wireless telephone antenna, an automobile wireless telephone antenna, etc. and more particularly to an improvement in a frequency band broadening means for the antennas.
2. Prior Art
It is generally necessary that ultrashort-wave antennas used in wireless telephones of the types described above are compact and light-weight and also that the antennas have broad-band characteristics. However, as the size of the antenna is reduced, the width of the frequency band for the antennas generally becomes narrower. Currently, however, research and development work that is aimed at digitalization of wireless telephone systems has progressed greatly. As a result, the usable frequency band has been expanded from the conventional band width of 860-940 MHz (80 MHz) to a new band width of 810-960 MHz (150 MHz).
One way to improve the broad-band characteristics of the antennas is TO set first and second conductive substrates to face each other in a parallel arrangement, thus forming a 1/4-wavelength resonator and install a 1/2-wavelength antenna element on the first conductive substrate. With this structure, a double-tuned circuit is formed by the parallel resonance circuit of the 1/4-wavelength resonator, and the broad-band antenna characteristics are thus obtained. Antennas obtained pursuant to this method show broad-band characteristics that are considerably improved over those of previous antennas.
Conventional wireless telephone type ultrashort-wave antennas AS described above, however, HAVE problems. The electrical length of the second conductive substrate is about a 1/4 wave-length; accordingly, if the frequency used is 900 MHz, the overall length becomes approximately 80 mm. Furthermore, in order to lower the "Q" value, it is necessary that the width of the second conductive substrate be approximately 20 to 25 mm. It is further necessary that the 1/4-wavelength resonator be approximately 8 mm in thickness. Accordingly, even if a loading coil is used to reduce these dimensions, the matching part of the second conductive substrate has a length of 45 to 50 mm, a width of 12 to 13 mm and a thickness of 6 to 8 mm. Thus, it is extremely difficult to reduce the size of the antenna and at the same time to obtain the broad-band characteristics in the antenna.